1. Field of the Invention
The present invention relates to a method and an apparatus for measuring a thickness or grammage (mass per 1 m2) of a sheet-like substance including paper, non-woven fabric, and film by utilizing resonance of a microwave.
2. Description of the Related Art
The thickness and the grammage of the sheet-like substance including paper and film are one of the most important characteristics in product quality of a production process. In the production process, it is necessary to perform online measurements of the thickness, particularly the grammage in a case of the paper.
Conventionally, a method in which a beta ray is utilized has been used as the method of performing the online measurements of the grammage in producing the paper, and particularly, krypton-85 is mainly used. The beta ray is a kind of radiation and is a charged particle generated by beta decay of a radioactive isotope. One of the surfaces of a sample is radiated with the beta ray, and an amount of beta ray transmitted through the sample is detected at the other surface. The beta ray with which one of the surfaces of the sample is radiated, excites states of atoms constituting the sample, or ionizes the atoms in the course of the beta ray through the sample. The beta ray also experiences energy loss by losing kinetic energy of the beta ray in itself, and then the beta ray is transmitted through the other surface. The amount of energy loss is determined by physical properties, such as an absorption coefficient, and an amount such as a thickness and a grammage, of a substance constituting the sample. The energy loss is expressed by Formula (3).I=I0 exp(−μρχ)   (3)
where, I0: radiated radiation intensity
I: post-transmission radiation intensity
μ: absorption coefficient determined by radiation energy and sample
ρ: density of sample
χ: thickness of transmission substance
Because of b=ρ·χ when the grammage is expressed by “b”, Formula (3) can be expressed by Formula (4).I=I0 exp(−μb)   (4)
That is, the absorption coefficient μ which is determined in each sample can be determined by computation, when a calibration curve is previously made in each sample which is of a measuring target using a standard sample whose grammage is known. Therefore, during the operation, the grammage of the sample can be determined from Formula (4) by measuring the radiated radiation intensity I0 and the post-transmission radiation intensity I. Thus, in the method in which the beta ray is utilized, although it is necessary to previously determine the absorption coefficient of the target sample, the grammage can instantly be determined online during the operation, so that a paper machine can be controlled online.